An electro-optic material has at least two “display states,” the states differing in at least one optical property. An electro-optic material may be changed from one state to another by applying an electric field across the material. The optical property may or may not be perceptible to the human eye, and may include optical transmission, reflectance, or luminescence. For example, the optical property may be a perceptible color or shade of gray.
Electro-optic displays include the rotating bichromal member, electrochromic medium, electro-wetting, and particle-based electrophoretic types. Electrophoretic display (“EPD”) devices, sometimes referred to as “electronic paper” devices, may employ one of several different types of electro-optic technologies. Particle-based electrophoretic media include a fluid, which may be either a liquid, or a gaseous fluid. Various types of particle-based EPD devices include those using encapsulated electrophoretic, polymer-dispersed electrophoretic, and microcellular media. Another electro-optic display type similar to EPDs is the dielectrophoretic display.
Generally, an image is formed on an electro-optic display device by individually controlling the display states of a large number of small individual picture elements or display pixels. A data pixel having one or more bits defines a particular display state of a display pixel. A frame of data pixels defines an image. Commonly, the display pixels are arranged in rows and columns forming a display matrix. An exemplary electro-optic display pixel includes a layer of electro-optic material situated between a common electrode and a pixel electrode. One of the electrodes, typically the common electrode, may be transparent. The common and pixel electrodes together form a parallel plate capacitor at each display pixel, and when a potential difference exists between the electrodes, the electro-optic material situated in between the electrodes experiences the resulting electric field.
An active-matrix display includes at least one non-linear circuit element, such as a transistor, for each display pixel. An exemplary active-matrix display pixel includes a thin-film transistor having its drain terminal coupled with the pixel electrode. The gate and source terminals of the transistor are respectively coupled with a row select line and a column data line. To change the display state of the display pixel, the common electrode is placed at ground or some other suitable voltage and a row driver circuit turns on the transistor by driving a suitable voltage on the row select line. An optical-property-dependent voltage corresponding with a display state transition may then be driven on the column data line by a column driver circuit.
An electro-optic display device may have display pixels that have multiple stable display states. Display devices in this category are capable of displaying (a) multiple display states, and (b) the display states are considered stable. With respect to (a), display devices having multiple stable display states include electro-optic displays that may be referred to in the art as “bistable.” The display pixels of a bistable display have first and second stable display states. The first and second display states differ in at least one optical property, such as a perceptible color or shade of gray. For example, in the first display state, the display pixel may appear black and in the second display state, the display pixel may appear white. In addition, display devices having multiple stable display states include devices having display pixels that have three or more stable display states. Each of the multiple display states differ in at least one optical property, e.g., light, medium, and dark shades of a particular color. As another example, a display device having multiple stable states may have display pixels having display states corresponding with 4, 8, 16, 32, or 64 different shades of gray.
With respect to (b), the multiple display states of a display device may be considered to be stable, according to one definition, if the persistence of the display state with respect to display pixel drive time is sufficiently large. The display state of a display pixel may be changed by driving a drive pulse (typically a voltage pulse) on the column data line of the display pixel until the desired appearance is obtained. Alternatively, the display state of a display pixel may be changed by driving the column data line over time with a series of drive pulses regularly spaced in time. In either case, the display pixel exhibits a new display state at the conclusion of the drive time. If the new display state persists for at least several times the minimum duration of the drive time, the new display state may be considered stable. Generally, in the art, the display states of display pixels of LCDs and CRTs are not considered to be stable.
EPD devices may be used in many different applications. For example, EPD devices may be used in electronic readers, cellular telephones, digital photo frames, and commercial signage. In various applications, the EPD device may be used to render a main image. Considering the example of the electronic reader, the main image may be a welcome screen, a page of a book, newspaper, magazine, or other document. In addition, the EPD device may be used to render an “overlay image.” The overlay image may be, for example, a pop-up menu, dialog box, icon, cursor, battery charge level indicator, message indicator, text, or other type of graphical image. The sub-windows may appear to overlay the main image being rendered. The location and size of the overlay image on the display may vary. During a session of use, a variety of different main and overlay image may be rendered at different times.
The display state of an EPD display pixel may be changed by applying one or more drive pulses. The drive pulse(s) required to change the display state of an EPD display pixel may depend on the prior display state of the display pixel, as well as other factors, which presents novel problems when rendering overlay images on an EPD device.
Accordingly, there is a need for methods and apparatus for efficient rendering of overlay images in an EPD device.